1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device for nonvolatilely writing data in accordance with application of a voltage to a variable resistor.
2. Description of the Related Art
In recent years, attention has been focused on a nonvolatile memory comprising memory cells each containing a variable resistor, which are connected at intersections of word lines and bit lines and arranged in matrix.
Known examples of the nonvolatile memory of such the type include: a PCRAM (Phase-Change Random Access Memory) that uses a chalcogenide element as the variable resistor; a ReRAM (Resistance Random Access Memory) that uses a transition metal oxide element; and a CBRAM that changes the resistance by precipitating metal cations to form a bridge (conducting bridge) between electrodes and ionizing the precipitated metal to destruct the bridge. These resistance memories are characterized in that the variation in resistance is stored as information.
The PCRAM utilizes the shape, such as the magnitude and the width, of a current/voltage pulse applied to the chalcogenide element to control the process from heating to cooling, thereby causing a phase change between the crystalline state and the amorphous state to control the resistance of the element (see Patent Document 1: JP 2002-541613T). The ReRAM includes the bipolar type and the unipolar type. In the case of the bipolar type, the direction of the current/voltage pulse applied to the transition metal oxide element is used to control the resistance of the element. On the other hand, in the case of the unipolar type, the magnitude and the width of the current/voltage pulse applied to the transition metal oxide element are used to control the resistance of the element.
The unipolar type is preferable to realize a high-density memory cell array. This is because in the unipolar type a variable resistor and a rectifier such as a diode can be stacked at each cross-point of a bit line and a word line to configure a cell array with the use of no transistor. Such memory layers can be stacked to increase the memory capacity without increasing the area of the array. This is the purpose of a three-dimensional stacked resistance memory.
In the case of the ReRAM of the unipolar type, data can be programmed in a resistance memory by applying a program voltage of around 6.0 V to the variable resistor for around 10 ns, thereby changing the variable resistor from a high-resistance state to a low-resistance state. This state change is referred to as “program” or “set”. When an erase voltage of around 2.0 V is applied to the data-programmed variable resistor and a flow of current of 1-10 μA is supplied for 200 ns to 1 μs, the variable resistor is changed from the low-resistance state to the high-resistance state. This state change is referred to as “erase” or “reset”.
The resistance of such the variable resistor can be read out by applying a certain voltage to the variable resistor and sensing the value of current flowing in the variable resistor. Application of a voltage of 0.5 V to the variable resistor part requires application of a voltage, 0.5 V+Vf, to an actual bit line, which additionally includes a voltage Vf corresponding to the loss in the diode. This bit line voltage is generated by applying a clamp voltage to the gate of a clamp transistor in a sense amplifier circuit. The clamp voltage is generated from a bit line clamp voltage generator circuit.
The diode loss Vf has temperature dependence. Accordingly, if the clamp voltage and finally the bit line voltage have no temperature dependence, the voltage applied to the variable resistor part has temperature dependence instead. Therefore, even in the same variable resistor, the current flowing in the variable resistor part varies depending on the temperature and shifts the 1/0 decision point on sensing, which reduces the sense margin as a problem.